Unity 'Hold On' upon launch goes forever

I downloaded an asset from the asset store, and it warned me that it was made for Unity 4 and may not work with unity 5. I was fine with that, since I've worked with those types of assets before and nothing had went wrong. So after downloading it I then tried to import it, but it got stuck on some lightmaps and the 'Hold On' screen was going on forever.

I closed out of Unity and tried to reopen my project, but that Hold On screen keeps coming up. It always gets stuck on Lightmaps and I can't even access the project I want. Task manager says the program isn't responding. How do I stop this asset from importing? How can I access my project?

Thanks!