Yes, the resets you are referring to are built into the combination thermostat/ hi-limit emergency cut off. These are on the face of the water heater so to speak.
Since you have a dual source water heater (propane and electric), that operate independently of each other, then you also have two thermostats/eco's. One for the propane/dsi and one for the electric heating element.
The thermostats are what cuts power to the water heater (either 12 volt DC or 120 volt AC when the water has heated to a certain preset temp. This is just like any thermostat in say your furnace, air conditioner, etc. Usually this is around 135 degrees for the water.
Now the emergency cut off (ECO) or hi-limit cuts the power when the temp get too high in the tank, as when the thermostat failed to cut off the power, or possibly another reason, like no water in the heater. This is designed to keep from burning up the wiring and I think the ECO trips around 160 or so degrees. if the ECO trips, you have to manually reset it, before power can once again flow to either your 12 volt gas solenoid, if that is the eco that trips....or your 120 volt electric heating element, if that is the ECO that trips. Both may be tripped if something has happened that would allow the temp inside the heater to get to 160 degrees.
To reset one or both of these ECO's, go outside and access your water heater. The ECO's are covered with a rubber plug. All you have to do it to press the resets on the ECO's thru the rubber. That's all. No need to remove the rubber cover, unless you have to replace the combination thermostat/eco. If I recall, the left hand one is for the electric, and right hand one is for the propane...but don't quote me on that.
Once you verify the hi-limit ECO are not tripped and you manually reset them, you can once again go inside and try to start the propane with your DSI switch and see if the propane lights up this time. It usually takes 15-30 seconds after you turn on the DSI switch before it ignites. You will hear a rapid clicking noise at the water heater as the DSI is making sparks to ignite the propane. If it lights, then the burner comes on the water heats up.
If it fails to light after a few seconds, then the DSI can attempt to light it again after a few seconds, and you can just wait to see if you hear it clicking again. After usually 3 failed attempts, the the DSI goes into a lockout mode. It will not attempt to light again, until you turn the inside switch off. then on again to start the process anew.
Not all of the older Suburbans would try 3 times though and yours may lockout in less than that.
If you look at the pic above, the DSI electrode that makes the sparks, is at the end of that orangish wire in the bottom right hand side of the pic where the burner tube goes into the water heater..