Chaos. Absolute mind-fu**ing Chaos.

That's what will happen. If Modi loses, then BJP won't be able to form the government at center. There are two alternatives to this.

1. UPA government:- Let's get the elephant in the room out of the way first. Rahul Gandhi is going to be the PM of India. Say it out loud. Say it out loud until you're convinced or you roll on the floor laughing, whichever comes first. That's not the biggest reason this is bad. Worse, he's gonna go to all other countries, economic forums, UN meetings representing India and giving speeches about India. Now we all know how good he is at delivering speeches. Imagine him saying something like “Politics is in your pants, shirts and pockets”. We'll be the laughing stock of the world. Much like the world laughs at Trump now, they'll laugh at India. Apart from this, I can confidently say we're going to see a repeat of UPA government again. In 2004, we're in prime position to become a superpower. We became a nuclear power, infrastructure has been improved a lot, investments were flowing in and the world was looking at India and it all came down crashing in 5 years. In the beginning of UPA-1, it was fine. We even signed the nuclear deal with America and grew at a healthy pace. Then it all came out, the appeasement politics, the pseudo-secular stuff, inflation became high, growth slowed, lives broken, dreams broken, hell, even states were broken without any planning to suit the political needs of one family. If you doubt this, look at the current Congress leaders. There are no new faces. It's still Manmohan Singh, Chidambaram, Digivijay Singh, Kapil Sibal and all the old guard who are alive and kicking. And they've their allies who are no less. NCP, DMK, YSRCP, Mamata etc. Kejriwal may also join this coalition. Do you want to be ruled again by these guys? The man who says people of one religion have the first right to resources of the country over others?? The lady who plays politics against Durga Puja in Kolkata?? The lady who closed all RSS schools in Bengal but not Madarasas( it boggles my mind that she used religion as a reason to do this). India would become hell and communal violence would go 10 times higher because that's what these people feed on and thrive. Development will take a backseat and we'll be back to appeasement and religion/caste politics all over the country. And that's not even the worst case scenario.
2. Third front coalition: I don't have to tell you why this is bad. All the parties with different agendas will pull India in so many different directions until it breaks. There is no Jayalalitha now, Chandra Babu Naidu is not the force he once was, Nitish Kumar is struggling to win Bihar and we have Kejriwal and Mamata here again, the loudest of the bunch and the worst. Too many egos and agendas clashing with one another. However, this is not a probable or even a possible scenario, as BJP is ruling many states in India. Even the voters seems to prefer a national party at center instead of local parties due to the bad experiences in the recent past. But, politics is an ever changing game and we'll have to wait and see.

I'm a BJP supporter but not a Bhakt. BJP and Modi have their own faults in both religious and governance lines but, I think they've their priorities right. This government had shown the willingness to push reforms and remove red tape with good governance and economical growth in mind. Modi is also becoming an important and acclaimed leader across the world as seen in the Davos economic forums recently. Say what you want about his speech, but he's the first prime minister of India to give the opening speech and that speaks volumes about the standing India gained in the world recently. This government deserves a second chance and will most probably get it, for the lack of a better choice if nothing else.

Edit: I totally forgot about the support given to our army by this government. That's one of the major changes from the previous government where we were always on the defensive and the army was not allowed to give a befitting response to Pakistan. Remember when a soldier's head was cutoff and the army was not allowed to do anything until there was a media uproar and public outrage over it? That's definitely gonna happen again. Some of the other answers covered it well. Just wanted to add my two cents here.