Sell it for parts on craigslist (don't expect much), or recycle it (please don't put it in your garbage)
Other than that, any computer store should be able to diagnose it without much cost, could be the power cable/supply but regardless old gateways are known for unreliability, so even fixing this will just lead to needing more money invested.
I'm writing this without even looking up your systems specs, so if it means a lot to you, or you think its got potential to be of good use to you please follow up on it, it might not need too much!