Originally Posted by HillRider

I's almost certainly a Riv-Nut or equivelent so try tightening the existing one. These things are a bit like Moly anchors

Get a moderately long M5x.8 bolt, say 25 mm long or longer and a matching nut. Any decent hardware store or home center will have them. Thread the nut up the bolt and then thread the bolt well into the loose water bottle cage fitting Hold the bolt head stationary with a screwdriver, allen wrench or what ever tool is appropriate and thread the nut down against the top of the loose fitting. Then tighten the nut firmly against the fitting using an open end wrench.

That should expand the fitting and lock it back in place.

I've also heard of the same procedure being done using a qr skewer and a piece of small diameter pipe as a spacer. Put the spacer on the skewer and thread the skewer into the fitting until the spacer is snugly trapped between the fitting and the cam-lever end. Then carefully close the skewer lever to compress the fitting and tighten it up.

The nut and bolt technique is easier.