Siemens S7-300 CPU314 AND TP270

I am new to this plc, and I have a problem with communication betwen the hmi tp270 and the cpu314c-2dp using mpi port.
Do i need to set the terminating switch resistances "on" in both ends?
What might go wrong that the communication is lost, it had been ok for the last few months. I inherited this project recently and I have no backup of files

Yes, you have to terminate the MPI bus at both ends, if both the CPU and the TP270 are at the physical ends of the MPI bus!

If you get a BF led blinking on the CPU, then you have accidently inverted the A and B wires on one device (edit: or perhaps a shortcircuit). If the led lights up continuously, then you probably have a broken wire or a bad connection.

Edit: There is, of course, always the possibility that someone has been tampering with the TP270. If you enter the Control settings at power up, you can easily change the MPI address or the transfer speed of the TP.

So what is the nature of the problem? Does the panel draw correctly but you don't get any interaction from the HMI? Do you have any fault lights on the S7-314? Is the PLC in run mode? Check the status LEDs and also look at the little three position switch, it should be up in the RUN mode. The 314-2DP has two communications ports, the MPI port (left hand connector) and the Profibus port (right hand connector). The TP270 port can be configured for MPI or Profibus from the Windows Control Panel. You need to connect to the same type of communiations port on the PLC. Is it possible someone moved the cable from one port to the other? Is your communications cable damaged?

MPI Address

Lets start with the basics....Do you have the touch panel defined with a MPI address in the HMI and hardware config? If so if goto the PG/PC interface and test the MPI network, do you see the PLC and touch panel address checked? Do you have MPI enabled or is profibus or serial enabled on the touch panel? Did you define the MPI address on the touch panel?