Is there anything in her cage she can climb on? Sand boas are sucky climbers.. there's almost no way she could get up very high, unless she had something perfect to climb on.

Are there any gaps anywhere in the cage? Are you SURE she's not in the substrate? They are excellent at hiding..

When my sand boa got out (only escape I have had) he only went about two feet away, was hiding under the curtains. Sand boas aren't terribly active, so look around the immediate proximity.

Are you sure no other family members had her out or anything?