What do the lights say? That would go a long way toward helping us help you.
My uninformed guess would be a problem with suction control or the door switch. If you didn't reassemble to suction control valve parts properly, then it may think the bag is full. Or, if you didn't reassemble the bag door correctly, it may think the door is still open. Either scenario would cause the vacuum to not run. Not knowing what the lights are saying, I'd check those areas first. If no lights are on, then there could be an issue with the power supply (wires, connections, etc).
