I recently updated a couple of Catalyst 2960X switches at work, going from the previous Cisco-recommended IOS version to the current, 15.2.4E6(MD). Most of the switches updated just fine, but a 2960X-24TS-L failed to come back up after installing the new IOS image and reloading.
I tried power cycling the switch and watched the boot process in the console, where everything looked pretty normal until it got stuck on the thermal/fan test (POST: Thermal, Fan Tests : Begin). The SYST LED was flashing at this point and the switch's fan was at full speed. Time to google.
One post on Cisco's forums suggests power cycling the switch (which I had already tried), and another suggests checking out the solution for Cisco bug CSCtx90729. CSCtx90729 doesn't actually mention any switches from the 2960-X series in the list of affected devices though, so I had no way to know which image (if any) incorporated the fix for this bug. I picked 15.2.6E1 since it was the highest available IOS version, used a USB drive to copy it onto the switch alongside the 15.2.4E6 image and booted 15.2.6E1. The switch came up with zero issues, so I set the 15.2.4E6 image as the boot image, crossed my fingers and reloaded... and it worked.
I honestly have no idea if booting 15.2.6E1 actually fixed anything or if the switch just needed a bit more power cycling, but at least I didn't have to open up a TAC case and waste more time on the issue, so that's nice. I'm also really starting to appreciate the addition of USB ports on Catalyst switches. They're a fantastic way to quickly get an IOS image onto a switch if you're stuck in the bootloader.
