You need to find out why that hapened. There is no reason for what that to happen. Without turning on the A/C you should easily be able to run on a 15 amp circuit with an extension cord ( a good one). We have done it many times and it is all I run on at our family cottage with a 100 foot extension cord. We can even run the A/C on low as long as everything else is off.
On the other hand when I built my new garage I put a 30 amp RV plug outside (for use by us or any visitors with an RV) and one inside in case I need to make repairs or have a service person come by.
It is an easy upgrade at your home and relatively inexpensive as you do not need outdoor or underground wire so long as it runs from your panel directly into a sealed outdoor box.