Laptop won't turn on after disassembly, fan cleaning and reapplying thermal paste

[DBrix1]

I'm seeking help for my laptop which doesn't turn on. This is after I disassembled it and clean the fan, also re-applying a thermal paste as its temp reaches too high to the point of frequently shutting down.

Upon my first re-assembly, the laptop won't turn on, without display even with external monitor through HDMI. The LEDs seems to work at first boot, the power LED and WIFI LED lights turn on. The fan is also spinning.

However, the second time I tried to power it, only the power LED is left and the fan is no longer spinning.

I am pretty sure that I did not leave any ribbon connectors unplugged as the only thing I unplugged was the keyboard connector in addition to the heatsink fan. I also did not remove my processor as I am aware of the delicacy of the pins.

I tried plugging in my mouse or keyboard to see if it also receives power but with no luck. I also tried removing, alternating, and replacing the RAM modules as I read that it may be the cause, also tried it with my hard drive. Removing the CMOS battery and re-inserting also didn't fix it.

I've read on some forums that it is also possible that electrostatic discharge might have damaged it. Is it possible that I damaged my motherboard while cleaning it? I used a simple brush to remove the dust build up and a hair blower. This is the first time I tried to disassemble a laptop and clean its fans and thermal paste so I have no idea what might have damaged it.

I'm looking into sending it to a technician to troubleshoot it but the cost of service in my country is expensive, plus I can't really go out due to the quarantine.

My Laptop specs:

Toshiba Satellite C850, i7-3630QM with AMD 7610M, 6GB RAM, 1TB HDD

[DBrix1 · Accepted solution]

UPDATE: I got it to boot now. By removing my CMOS battery and let it power on by plugging in my AC, it goes into my bios now. Re-inserting my CMOS battery and booting up works fine now.

I do have a question in updating my bios. Currently my installed bios version is 6.10. I looked for updated drivers in toshiba/dynabook drivers site and found that there is 6.80 bios version available. Upon launching the application, it says 'insert battery' then quits suddenly. My battery is long dead now as it is already 7 years since I bought my laptop, I've been using it with the AC since then. How can I update my bios to latest version without inserting battery?