I suspect a bad keyboard.
Sounds like a stuck key. Keyboards for desktop PCs are relatively inexpensive - like $14. For Windows notebooks - varies, and difficulty replacing them varies. For Mac systems, just depends as some Macs are not easily serviceable if a Mac notebook. Would you like to continue discussing this issue with me? --remoteITpro
What kind of system? Desktop? Notebook? Windows? Mac? The keys are really not serviceable or repairable on about any keyboard these days.
Just to eliminate the keyboard, do you have a wired keyboard and mouse to try? There may be a youtube video available on repairing just a key, but there would be no guarantees on anything like that.