When I woke up this morning, suddenly my filters disappeared. All of my songs show up on the bottom, but I am unable to sort them by the filters. Nothing was changed last night. Can anybody help me out?
(http://i.imgur.com/mdtcSYb.png)
Go to Preferences -> Columns UI (that's what's displayed in the picture you uploaded) and switch to the Filter tab.
You need to have the appropriate fields set there. E.g., you have set Name - Field = Album - %album%.
Check the folders and their statuses at 'File/Preferences/Media Library'. Make sure those still exist and are reachable. Re-add them if necessary.
My bet would be that you are using an external HDD and its letter changed for whatever reason.
I followed Daeron's advice, and it worked! Thanks!