Why doesn't midi input work

Hi,
I'm trying to get my Alesis v25 midi keyboard to work with musescore. I've already succeeded at doing so; however, it has stopped working. I did what I always do: log in to my computer, plug in the midi keyboard, and then start musescore. I went into Edit > Preferences > I/O and made sure that it was set to "MME" and "MMSystem,V25", and nothing worked. I've restarted Audio and MIDI devices many many times. Anyone got a solution?