My HP ProBook 4520s fan is really loud and seems to be running a lot. My laptop is warmer than usual too.

It sounds like you may have spilled something on the key causing it to stick. Dampen a cloth with mild detergent such as liquid dish soap and wipe the keyboard. Then wipe again with a dry cloth. If this doesn't work, clean underneath the key or replace the key altogether. Instructions for removing the keyboard for cleaning or replacement can be found in the keyboard replacement guide. General tips to resolve other issues can be found in the troubleshooting guide.