I have been using Toshiba l635-137 for almost 8 years without any issue with recently updated to windows 10 64bit and it has core i5 processor with 6 gb ram, day before yesterday while working in excel the laptop display just set scrambled and blacked out and shut down and started restarting for 2-3 times and finally stops down.
when I press the power button, the power light lights up while the hdd light the orange one lits up for few seconds and goes off.
Kindly advice what could be the issue
Sounds like overheating to me. If it is just mild, give it time to cool down and then see it if will turn on. However if it has been overheating for a while, then it could have fried something.
If it does in fact turn back on, then monitor the temps for the laptop. But if it won't come back on, even after having time to really cool down (a couple hours) then I would suggest taking it to a local tech as you likely have parts that are going to need replacing.