I have no idea what brand it might have been. And really the brand means little.
From your two photos I see 90s hybrid bike brazed with lugged steel (not welded like I originally said) - you can see the bronze around the bottom bracket.
Dunno if the flat bars are original... the single rear derailleur looks a bit odd mounted on the quill stem. Normally they'd have thumb shifters. And that orange shift cable outer looks decidedly weird looping up at the back chainstay.
The wheels may not be original either. There appears to be a relatively large tyre on both wheels, at a guess 32mm or 35mm ? The mud/colour makes it hard. Schrader valves imply larger tyre widths too. Quick Release skewers also implies no older than the 90s.
There's a bulbous bump in the seatpost - does the bike have seatpost suspension?
There's an odd plastic thing on the seat pole just below where the stays are attached. Not sure of its function. What is it?
The crankset looks relatively modern - I suspect this has been converted to a single chainring at some point.