If the polarity of the external power supply is reversed, it is possible that the amplifier chip is burnt.
If it is now turned on correctly, all functions will work but there is no sound.
The amplifier chip
(LA4192) will need to be replaced.
You can find it here:
ebay co uk LA4192
Therefore, it is very important to check before turning on the external power supply.
There should be information next to the jack - see picture below.
Usually, old cassette players have a negative center, and the adapters have a positive center. Read more here:
plug polarity