Each Casino has it’s own policy regarding “connection lost” but for the most of them, these policies will be quite similar although it can very depending on each game.
For example, if you’re playing slots and you have placed your bets but have not yet spin the reels at the time when you lost the connection, chances are that the session will simply terminate as if you had just click “Cash out” and left the table.
If you were playing slots and the reels are already spinning when you lost the connection, then it’s likely that the casino will first have a look at your session before making a decision. This is because there are players that will try to avoid a losing hand by manually disconnection their computer from the internet and blame it on losing the connection.
Other games like poker, gin rummy, backgammon and other multi-player games will have a “time bank” that will give you a reasonable time to come back to the table in if you suddenly lose the connection.
Online Casinos are very well aware of potential connection problems and overall they will have very reasonable policies about it.