There are some cases when the player heals himself or others (FAK / Medikit) and the healing got cancelled by an other animation. After this, the player stays at the same health level and can't heal himself up again (edit: actually can't use any other object related action). You also can't get into vehicles. (It may has more effects than these ones) In my experience, respawning resets this bug.
EDIT: The same happens if you heal other players/AI: I edited the Summary and the descriptions according to that. Also changed the category from Health System to Anims.