It could be the high voltage fuse that's blown, or maybe the magnetron has failed. Both are relatively easy to replace.
My microwave stopped working (same as yours, it appeared to be working but would not heat food). My landlord bought me a new one, but I preferred the older one so I fixed it myself. Mine turned out to be just a blown HV fuse which was only 43 baht to replace:
https://shopee.co.th/ฟิวส์-กระบอกฟิวส์-ไมโครเวฟ-เตาอบ-Microwave-Fuse-5kV-0.65A-0.7A-0.75A-0.9A-สายกระบอกฟิวส์-MW-ขาว-(1-ตัว)-i.253932797.22473681910
If it's the magnetron, they are also not expensive to replace (I bought one before I knew it was a blown fuse):
https://www.lazada.co.th/products/i3076807555-s11297855015.html?urlFlag=true&mp=1&tradePath=omItm&tradeOrderId=740461231503272&tradeOrderLineId=740461231603272&spm=spm%3Da2o42.order_details.item_title.1