Windows 10 - inaccessible boot device after cleaning and "attempting" to replace heatsink

[Sevgi]

Today I decided to clean out my computer using compressed air and replace my old heatsink with a new one. Put some new paste on but the idiot i am i couldnt install it, couldnt click those four things in place. Decided screw it and try again another time, put my old one back in but now when i start it up i get that error screen with the big :( and gives me the message "INACCESSIBLE_BOOT_DEVICE".

How would i fix this and what would I of done for this to happen? Thanks, first time ever doing something like this.

[editline]29th March 2016[/editline]

Forgot to mention sometimes ot turns off as soon as i turn it on with a single short beep.

[Sevgi]

Made sure all the cables were plugged in correctly.
Tried changing from IDE to AHCI, etc
Still nothing

Didnt think i could fuck up so hard replacing this thing and cleaning it.

[mastfire]

have any external devices plugged like flash drives,phone etc?

[Sevgi]

Only had mouse and keyboard plugged in really so yeah.

Spent most of yesterday trying to find a fix and said fuck it and took it to get repaired tbh. Thanks anyway tho, im a tard.