Hi
If you are not sharing your account you are fine. Probably you got "somebody else is using this account" because you didn't time out from the server when you lost connection, so when the system tried to reconnect you to the server you were still logged in.
Steam change password.
Try to logout and after a few minutes login again. Make sure Steam and TruckersMP are both closed after logging out. The system may think you're still in-game, so wait for like ~3-5 minutes to be sure.
İf you have still problem please make a ticket to supports :https://support.truckersmp.com/