The engine won't run well and - depending on how much wrong fuel you put in - the diesel engine could end-up badly damaged if try to run it more than a few seconds.
Unleaded gas is simply not suitable to run a diesel engine. Engines which run on unleaded use a much lower compression ratio than diesel engines.
You need to drain the tank and the whole fuel delivery system and then refill the tank with diesel.
For more information please see the answer to the Related Question shown below.