This actually happened to me on PC. I was playing on Insanity so I was fully commanding my squad who were Kaidan and Garrus most often, specced into their tech abilities to compliment an Adept Shepard- I was always running tech companions so not certain whether other abilites would set this off but it was bizarre but definitely triggers with keyboard and mouse. It would be anywhere up to a few decent heartbeats where I wouldn't be able to shoot but do everything else.
It wouldn't frustrate me until the end of combat, however this as an Adept with absolutely no gun skills. I didn't even spec into Pistols because I could use everything.
If that helps, be aware of krogan in ME1 as they use stasis on you. It is possible that some enemies are using biotic or tech powers which disable your gun not only the use but switching too.
