Philips 21ST2430/10B - No picture/No sound

Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

FX transmitter

If you need further help, I’m available over the phone at https://www.6ya.com/expert/richard_b59056c0c62ecb00

Same issue with - Panasonic TH-50PX70A , power UP with red standby, then power ON by remote - only Gives GREEN led blinking 35-40 times, followed by solid green led with no no picture no sound.
>> Solution; unplug Plasma from AC 10-15min, then plug in & power on to Red standby led,, At this point - Power ON tv while holding down both -Volume Down and F (function Select) buttons (front panel door) at the same time turns picture ON,all ok.

• Five blinks - AKB circuit (automatic kine bias), the timer/standby indicator blinks for about 30 seconds then goes to the self-diagnosis function. Something is wrong with the video. Check video out, Q705, 732, 761 and other components on the C board, also check Q218, 219, 220 on the A board. In addition, unit could be in IK blanking, try turning up screen A2 slightly.
  

Switch the TV on: Press the power switch A on the front of the TV. A green indicator on the front of the TV lights up when the screen is on. If the indicator is red, press the
-CH + keys on the remote control.
The green light blinks every time you press a key n the remote control.

Ref. link:
https://www.manualslib.com/manual/173137/Philips-29pt9420-69r.html?page=4#manual

The remote control will not operate the TV when the TV is in the OFF mode (No LED lit on front bezel). To change the mode from OFF to STANDBY or ON, press the POWER button on the side of the TV. Once the POWER button is pressed, the LED will change to either red to indicate STANDBY or green to indicate ON, whichever mode the TV was in prior to being turned off with the TV POWER button.

By pressing the POWER button on the side of the TV, the mode will toggle from OFF to either STANDBY or ON. If the POWER button is pressed and then the LED on the bezel is red, the TV is in STANDBY mode and will need to be turned on using the remote control. If the POWER button is pressed and then the LED on the bezel is green, the TV is ON.

Thats the great sound of Exploding capacitors on either the power supply board. or another one in there. Now the set is in fault protection. It tried to warn you with the RED blinking LED that there was something amiss by shutting itself down. When you unplugged you reset the error and allowed it to do it again.

Best Regards and Smper Fi

first thing i would do is take the lamp out and look at it, it dhould be on the left side of the tv, you will see a little door at the bottom

projection bulb needs replacement

A qualified tech will need to practice visual acuity technique! Look at power supply circuit for signs of stress,dis-coloration and/or bad connects.Check for bad 47mf/25v cap in area of stress.Replace BC-327-25 xistor w/BC-328 and replace BC-337-25 w/BC-338,if present in your set's chassis style.