My screen is all blue, no image and voice. a lock on the upper left corner

It sounds that your TV is locked by someway. Just unlock it. Proceed the following.
Many Jensen TVs have a parental lock that can be set to stop children from watching certain television shows or channels. In order to activate or deactivate the controls, you have to know the four-digit code or parental lock passcode. If you forget the passcode or if the lock gets set accidentally, you can still unlock the TV.

• Turn on the TV using the remote control. In most cases, you will need the original Jensen remote to unlock the TV if you aren't sure of the unlock code. You can purchase an original replacement online if necessary.

• Look at the screen. If you see a red key on the bottom of the screen, press "Function" and "Mute" on the remote at the same time. This will unlock the channel. Do this if you want to bypass the lock to watch a certain show or channel while leaving the TV locked. All other channels will remain locked and the TV will lock again when it is turned off.

• Unlock all of the channels on the TV by entering 0711 or 3308. These are the Jensen factory reset codes that will reset the TV to the original factory settings. By doing this, you will loose any information that you have stored in the TV, such as channel labels and schedules. You will have to program the TV again by going through the entire initial set up.

•Call the technical support service number in your owner's manual if neither of those codes work. A technician can help you troubleshoot the television as the problem may not be the parental lock feature. Have the model number and serial number of the TV available when you call.
• Take your television to a TV repair shop if neither of the codes work and the TV locked by itself while it was turned on. In this case, the TV may need to be rewired.