How do i fix thomson scenium tv the standby light turns red but won't switch the screen on

I do not know specific on this model (which youdid not give a model number)

The most possible cause is capacitors which fail but also other parts or software issues can be at fault. Sets like most modern types have a start up routine which verifies sections of you set are working correctly before fully powering up, so if a fault is found just shut down.
There may well be a known part or parts but sets are made with components fitted by passing through "ovens" to make solder joints, these are at higher temperatures than normal tin/lead and standard irons are mostly not able to melt it without damage to track etc.
My advice is to contact Thomson (French electronics giant) for their nearest service agent a far better solution than to try yourself, many of the "thin" sets after a couple of years (out of guarantee) are not fixable because they require panel changes which soon run out of stock if many go wrong, sad but true.

Hope this helps........