Mac doesn't start after installing SSD
I followed the guide to install an additional SSD in my iMac (27'', mid 2011). After I put everything back in place, it won't start. When I press the power button, the fans start to spin for a short time, then stop, then start again, and so on.
Is there any indicator to figure out what's wrong? I checked all connectors I disconnected when disassembling, they're all back in place.