hello I have a 2000 dodge Durango 5.9 and my SUV wont pick up speed went from 60 to 40 to 30 to 10 now just start then shut off my truck ran fine tell I let someone do my tune up what do I need to do

Can someone please tell me what I need to do my SUV been sitting for two months because nobody know what's wrong with it please

Check if the gap on the spark plugs are right, check if the wires are in right order, check if the cap and rotor is on right. Since it happen right after the tune up check if it was done right. If that is ok check if the gas in the tank is good even put a fresh gas in there with some fuel injector cleaner, load test the battery and check if the alternator is giving you over 14 volts like it should. If you have another ignition key for it try if it will work with the spare one, maybe needs to be reprogrammed. Start on these things.