I had a stereo installed, an as soon as I left.

It's very surprising that's all you lost. GM cleverly designed the car so you can't replace a broken radio. You must get it repaired through the dealer and one of their two grossly over-priced repair centers. To address that lack of concern for their customers, some aftermarket stereo companies came up with "radio relocation kits" that allow you to mount the original radio in the trunk and put a new one in the dash.

It is likely the installer had to disconnect the battery and that is under the hood. The Engine Computer needs time to relearn the sensor readings and engine operating conditions. That relearn starts to take place as soon as you start driving the car but it typically only takes a few miles before normal performance is restored. If the installer disconnected or changed something that affects performance enough to be noticed and forgot to put it back, the Engine Computer will detect it, set a related diagnostic fault code, and turn on the Check Engine light if the problem could affect emissions.

After the '94 model year GM stopped allowing us to buy radio service manuals because they wanted to lock up all the repair business for themselves. The only way to get a radio fixed was to go through the dealer and the two repair centers. With shipping two ways, removal, and reinstallation, the bill for repairing a cassette radio was around $450.00. Most people just bought high-quality aftermarket radios instead that cost half that much.

To combat that, starting with some 2002 models, GM built the Body Computers into their radios so you couldn't replace them. If you take them out you could lose the power windows, cruise control, chimes, etc. The relocation kits allow you to mount the original radio in the trunk, then you snip the speaker wires and run them to the new radio. You still lose the chimes but anything else that involves the Body Computer will still work.

A lot of their radios and computer modules also have to be programmed to the specific car. When one needs to be replaced you can not buy an inexpensive used one from a salvage yard. Instead, you have to buy a new one from the dealer and let them program it to your car. It's their many business practices and things they designed into their cars that are not in their customers' best interest but it's what keeps them profitable. They don't make much profit selling cars so they had to devise ways to get money from the owners after the sale.

As for the loss of power, if it hasn't come back by now, the first step is to return to the shop that did the installation and have them double-check their work. If they can't solve it, take it to the dealer. Everything they find will be documented on the repair order. If what they find had nothing to do with the radio installation, you will be stuck with the bill but the problem will be fixed. If it IS related to the radio, present that paperwork to the shop. If they are reputable they should be willing to pay for the repair. You have to go to that shop first though. If you just show up days later, they could argue that the car ran fine when it left their shop and you would have no way to disprove that.