I read somewhere that once monitors could be damaged by sending wrong signals and it was used as a plus point by the hackers.I want to ask that today,is there any way hardware can get damaged by the signals?
It depends on the hardware.
In general, these days, the answer is no.
Even in the way back when, it was all a bit of a beat-up. It used to be the case before multisync monitors that you needed to match monitor and video card - if they were wrong then the monitor would not sync. (Video signals in particular are all over the place, depending on resolution (width AND height) and frame rate.
These days all monitors / screens seem to auto-sync, and in any event the liklihood of damage was always pretty small. If the signals were electrically incompatible then perhaps damage might have resulted, but thats why all the different types of sub-systems use different connectors.
The other things you might need to consider are the common USB, serial, etc ports. In general these are designed to be tolerant of connecting up all manner of strange things without damage. And likewise, software can drive those ports without damage to the hardware as well. (The days of dodgy hardware that could in some cases cause large currents to flow if for example you did input and output at the same time on a parallel port - those days are gone about a decade or more ago).
Pretty much the only thing you need to be concerned about is things like probing hardware. For example, probing a serial port is fraught with danger. You might send down a magic sequence of bytes to see if your favourite device is attached (for example, your remote controlled car controller). Trouble is, that same sequence of bytes MIGHT cause some other device to malfunction or do something "weird". So for example, probing for your remote controlled car controller MIGHT do something horrible to the attached nuclear reactor controller that was attached. Now this is pretty unlikely but it can't be ruled out.
Example: years ago Windows NT used to probe for mice on serial ports, and I had a bunch of development equipment attached to a serial port spewing out all kinds of junk which was nice in a serial terminal program. But on machine boot about once a month windows would interpret this stuff as being a mouse, and happily run with 2 mouse feeds. The cursor would take off across the machine which acted like it had a ghost in it. Figuring out that turning off my hardware-system-under-development fixed it... that took a couple of hours!
Summary: In general you can't damage hardware that is plugged up and compatible. You might get strange things happening but the chances of actual DAMAGE (and smoke and flames, etc, or big repair bills) is small.
All that said, when something does not fit, perhaps its that way for a reason. Putting the wrong type of RAM in (and pushing REALLY HARD to make it go) is not a good move. Likewise I have seen the "D" connector used for a serial port plugged in 180 degress rotated. THAT TOOK SOME PUSHING. If things are connected that are meant to be connected, you should not do too much harm.
(And finally, as a guy once told me: if you can break it that easily, it was not designed right.)