Stop me if you've heard this one before.
Microsoft has released Service Pack 1 for SQL Server 2014. However, do not install it if you use the SSIS catalog (SSISDB). From the Release Services team blog:
Notice: The SQL SSIS team has found an issue with SP1 installation if SSIS catalog is present in the SQL Server instance.They are currently investigating this issue including possible workarounds and fixes. We have temporarily put the package download on hold and will provide an update with a solution.
And you can see what can go wrong in the comments on this Data Platform blog post. If you're not using SSIS catalog / SSISDB, you should be fine, but if you haven't already downloaded the file, it's been removed - so you are going to have to wait for an updated package anyway.
If you have already installed Service Pack 1 and SQL Server couldn't start, please see this post for a fix.
For now, it is build 12.0.4050, and contains all fixes up to and including Cumulative Update #5 (12.0.2456). In addition to staying away if you use SSISDB, DO NOT install SP1 if you are relying on fixes from Cumulative Update #6 or any COD updates that came out after CU #5. Relevant links: