The chain tightener isn't working

I presume you are referring to the “derailleur” which is the part of the drive train hanging below the rear axle, with two small sprockets. That moves the chain on the rear sprockets to change gears.

Since you haven’t actually described what is going on, so it’s hard to know what to suggest. However I’d make a wild guess and say there’s possibly some problem with the cable that operates the rear derailleur.

Let me suggest that you ask a friend who has a multi-speed bike to show you how the derailleur normally works, so you can look carefully while the gears are being shifted, to help you see what NOT happening on yours. Look first to see if the cable actually moves through the housing when you operate the shifter. If it doesn’t, lubrication might solve the problem, or more likely, the cable itself needs to be replaced. That’s not expensive, but would require a few basic tools.

Another possible problem is that the derailleur is fine, but the chain slipped off the rear sprockets and is jammed between the largest rear sprocket, and the chain, or maybe it’s off on the small sprocket side.

Reply back here when you have more information.