Window frame slipped on one side

Hi all One of our old rear windows has developed a gap at the top on one side – looks like the entire right side of the window frame has “slipped” down about an inch. Any suggestions on who I should call to deal with this – both category (window specialist, handyman?) or specific recommendations? We’re still relatively new to this whole house-ownership and don’t have a full rolodex of tradespeople yet. Many thanks Ben