I believe a crack would be leaking by now, so it's probably only scratched. If it is a crack, do not push on the glass or you could trigger it to break the whole panel.
Your best option is to replace it as soon as you can, since a deep scratch could easily turn into a crack and a crack is only going to get worse. Make sure you have an emergency container for your fish and maybe lower the water level to reduce the risk of loss and damage, but luckily replacing just the tank is not that expensive. If you have access to a glass cutter you could replace the panel with a new one, but most people would just turn it into a terrarium/reptile tank.