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It may be that it is 'broken' (in point of having defective hardware, it may be that the operating system on it is corrupted.
Can you turn it Off so that it stays Off? if so, do so overnight, first of all, in case it is merely suffering from a temporary case of zapped electronics e.g. from static discharge.
(If it doesn't stay Off, then it may have a jammed Power button. Or similarly be defective in the Power button being prone to trigger.)
Then it may also have a corrupted OS. If it is an Android phone, a factory reset might cure that... but, if you have data on the phone that you want to save, it would be best to copy the data off onto another device, first, before doing a factory reset.
If you want to try a factory reset, please let me know what model of LG phone it is, and I will give you the factory reset procedure.