That is good. Situations like this are usually salvageable. Now, before you do your next smoke with meat, do a couple of dummy runs with the smoker to try to get the temperature higher. A couple of things to try - in the order that I would try them...
- Were you relying on the lid thermometer? These are notoriously inaccurate. The temperature inside could have actually been a lot higher (or lower) than you thought. Invest in a good digital thermometer - like a Maverick ET-732/733 - so that you can know what the temperature actually is on the cooking grate.
- Make sure that you are using good restaurant grade quality briquettes as these give out more heat than charcoal over a longer period of time
- Make sure that the smoker is in a sheltered spot. Even a slight wind can seriously affect the internal temperature as it causes the smoker to loose heat quite rapidly
- Could you connect the propane so that it can be used to increase the temperature if needed?
- Wrap the smoker body with some insulation during the smoke. It needs to be fireproof and a welding blanket works well.
Hopefully by the time you reach #4 you will have solved the problem