Twete,
As GDawg said, jiggle the float in the tank. It is common that it will get stuck and give a false low-level indication. Check to see that the electrical plug is plugged into the coolant level assembly - it could have been knocked during the battery swap. If you haven't changed your coolant for a while, it may be a good idea to do it. In the short term, you could just drain the reservoir, flush with a garden hose, drain, and refill with fresh coolant. This will remove any solids in the tank. This is a temporary solution since the reservoir volume will eventually get swapped with the coolant in the system. The partial flush will reduce suspended solids so they won't cause the float to stick so often.
HTH,
Chris