a couple of options -
1/ If the water is still warm - cover the tank in a blanket/duvet etc to try and keep the remaining heat in.
2/ If the water is too cool - fill a coke bottle up with hot water (not boiling) and place in tank - Watch your tank temp until you are pretty much back to normal and the do as mentioned in 1/.
Finally - If its only overnight.... can you beef the temperature in the room up with a heater / turn your room temp up.. remember - the tank can never be any colder that the room it is in.
Overall - stay calm...its not as bad as it seems, and good luck.
as above. When it comes to getting a replacement depending on your tank size, think of getting two smaller heaters rather than one bigger one and use both in the tank.
If you make sure the cool down is a slow as possible (throwing a duvet over top) your fish will be ok. Remember to warm the water up slowly when you get a new heater.
The visitherm heaters are good IMHO, and a good price most places also.
Ok, don't panic Try getting an empty 2 litre bottle (ie coke or similar) Fill it with hotish water and screw the lid on tight then gently lower it into your tank. Replace as cools but do keep an eye on your guage.