Black rectangle on screen. A black rectangle appears on my TV screen (about 5 in. by 12 in. in

Question

A black rectangle appears on my TV screen (about 5 in. by 12 in. in size). When I press any button on the remote it disappears for a few seconds and then comes back on again, especially on news programs where we see a lot of subtitles. Sometimes it stays off for the rest of the evening when the TV is on for some time. The following appeared on the screen at one point:
ON T
P>pfa'<u'lu'u'