First of all I have a 99 Audi A4 1.8t B5 approx 130k on it...On my way home tonight I got in my car, let it warm up for roughly 3-4 minutes then got in and drove 15 miles. The temperature rose like normal to the middle and stayed there for sometime. At about 10 minutes into the trip suddenly my coolant light came on. I drove 2 more miles home (drove light and coasted down hills) and parked and shut the car off. The temperature gauge was a little over the middle when I got home. I popped the hood and to my surprise the top of the reservoir tank was completely sheared. There was coolant sprayed everywhere. The temperature that night when I first started my car before I left to come home was 0 degrees Fahrenheit and -1 when I got home.... Hopefully I did not do any major damage to any components in the vehicle. I'm reading other places and they are saying that it could be a blown head gasket... But since it was so cold could some of the anti-freeze/coolant froze and the pressure had no where to go so it just popped the top of the tank open? I know I should get a new tank, but shoudl I replace the head gasket as well?