Spindle motor?, does the spindle run freely?, in which case it's likely either the motor, the controller card, or a short in the cables that run between. Not very informative, I know, but it needs diagnostics to determine any further.

A bit of history - I converted my SX2.7 to CNC, but when I first bought it it ran for <5 mins then died completely. Repaired under warranty by Arc Eurotrade - brilliant service and they had the kit to diagnose motor and controller (had to return both to them for this). Might be worth a call to see if they're prepare to help out - but it'll cost! Also, they stock the replacement motors and controllers....again, it'll cost.

Once I'd converted to CNC I was trying to reverse-engineer the controller card and ended up blowing it up a second time. At this point I realised replacing the controller and motor with something that suited my purpose better was as cost effective as replacing the controller card alone. You might want to think out of the box to replace rather than repair.