This is not a normal thing to happen unless the device is being used in an extremely humid area, or alternatively has been used in a cold environment and then has been brought into a normal room temperature and an attempt made to use it straight away, as both these cases can cause this. However the only real thing you can do is with the aid of a hairdryer on its lowest heat setting blow warm air into the open cassette flap for about 20 seconds, leave it for about a minute then repeat the operation. Its advisable to leave the battery out whilst you are doing this and do not replace it until after about an hour or so (no sooner) as if that model has a dew fault memory (I am not 100% sure if it has) it might reset itself by then. If this doesnt cure it then it requires more detailed examination with the casing removed.