If I leave a match early (e.g. because I’m doing PvP for the Star Marathon but I get paired up against bots instead), my ships that were in that battle are still considered “In battle” and I can’t use them. Something like this really needs a timer that shows how much longer we’re not going to be able to use these ships.
I think that might be connected with the server issues. If they don’t reboot after 14 days, their performance and stability drops.
You need to completely exit the game, if on Steam/Gaijin Launcher, re-login, then you will be asked, if you wish to return to battle. Press Yes, then you will be fine.
I had the same issue today. I didn’t hear a ping sound, neither it was shown that I was in battle, but I was unable to replace ships or change modules, until I re-logged.
It should be less than a minute for ships to return in battle. Sometimes it might be more, that means server heavy loads. Anyway it depends on servers, so, there can not be timer for it.