I'm not sure if this has only happened to me but...

If you press and hold Y in (as in to select the type of grenade or change equipped equipment) you will randomly drop your primary weapon and not pick up another weapon and you will not be able to change weapons.

So basically you are left walking around without a primary weapon, cannot select a secondary weapon and cannot pickup any guns. My "status bar" still shows me as holding my MP7A1 but my actual screen shows no weapon and the area where the MP7A1 image would show is just blank.