Why wont tv turn on today worked fine yesterday? powers up but shuts right back off.

This is a general answer to your question:
Your TV is shutting down because it is consuming too much current from the internal power supply. The power supply has a protection cct
to protect your TV from further damage. If yo have any technical background, then you can follow these simple steps to troubleshoot your TV fault with the aid of a schematic diagram. Let us revise what you need for your job:
1- You need a schematic diagram for your TV model
2- You must be very careful when dealing with powered equippments
3- You need to prepare all the required tools to start your test and repair
4- You must be very confident that you are able to do the job using scientific steps. We all have done many mistakes but at the end we became experts in our fields. Please remember the fault is hidden somewhere and you are going to dig it out.

Let me start now:
-Disconnect any device connected to your TV
-Plug your TV to the Main
-Power on your TV, If the fault is still persisting then check the timer setting of your remote control or take the batteries out. Restart your TV again and if the same problem appear then we need to start our repair now. I personally advice you not to do any think if you don't know what you are doing, but if you want to learn, then do the followings without powering your TV.
- Disconnect your TV cable from the main
- Place and secure your TV on a clean wooden bench.
- Remove the back cover carefully
- look at the boards and identify which is which and compare it with the technical diagram of your TV.
- Identify the power supply section. Remember that 50 % of the TV faults are within the power supply and the rest lies in different area. Furthermore, many faults occurs due to bad soldering, sparks,loose connecters, metallic debris, excessive heat and bad ventilation.

Now if you have identified the power supply board(SMPS) which is very easy to find, disconnect the output jack and test each of the male and female jack pins with the ground. Use an AVO meter (put the selector to ohms) to check for any short cct with the ground. Remembr that there are some pins connect to the ground in both jacks. If you find any thing abnormal then compare your findings with the diagram. Alternatively, you can power your TV and measure the voltages at the output pins of the power supply and compare your readings with the service manual, but please be very careful. Again remember that switch mode power supplies (SMPS)
can work with a range of voltages 90VAC-240VAC, so if you have a step-down transformer from 220VAC to 110VAC, then it would be great and more safe when conducting your tests. In this test we can isolate the fault if it is in the power supply or in a different place. Also remember that your AVO (DC voltage) readings would appear higher than normal because we have isolated the load.

If the power supply was perfect then we are sure now that the problem is somewhere else, but in certain and limited cases some SMPS cannot provide the required power to the load due to internal changes and shifts in the values of the components in the loads and the power supply. In this case you need more experience to identify the source of the problem.

We have now finished from the power supply, however in the old TVs there is another power supply source which is the line transformer. This one is very simple and I am going to discuss that later.