The mirror glass is attached to the back plate with glue, the back plate is attached to the mirror mechanism at 3 snap points (circled in red) and the mirror mechanism is attached to the mirror housing at 4 points (circled in green). Your mirror may be a different design, but the attachment features will be similar.
Check the 2 screws at the bottom of the housing to make sure they are tight. Checking anything else will mean unsnapping the mirror from the mechanism, confirming all attachments, and snapping it back in place.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)