Did you go from 2.0.13.2 to 2.0.15? If you did the latter, you should understand that it is probably not good to bypass releases when upgrading. It's best to upgrade release by release eg. 2.0.13 to 2.0.14 to 2.0.15. Also, make sure to run any attendant upgrade scripts. There's a script (in the upgrade folder) in 2.0.14 that you must run to upgrade from 2.0.13 to 2.0.14. Likewise with 2.0.15. If these upgrade scripts aren't run, then significant changes that occur in the database will not be made. And, in recent releases, significant changes were made. If those changes aren't made, it heightens the possibility that you're site won't run as is.
Also, if you upgrade without finding out what's wrong you exponentially increase the risk that you will lose data. If the database changes and you try to reinject the old database data, you'll likely cause yourself greater amounts of misery. The changes may not be that great. On the other hand, they may.
But, that's just my two cents, pesos, etc.