Can a mantra deeksha given , which was stopped abruptly be restarted again ? Are there any Do's and Dont's to do so ?
I have been told by the Gurus that forgetting the Guru given mantra is the worst possible thing that can happen to the disciple (I think it's extremely rare that it actually happens). Abandoning the mantra (i.e. if you have not "followed" the mantra diksha) is also not good at all.
Lord Shiva warns in this regard as follows:
guru-mantra-parityAgAd-rauravam narakam vrajet ||
Abandoning the mantra results in death; abandoning the Guru results in poverty. And, by abandoning both one goes to a hell called Raurava.
KulArnava Tantram 12.52
I not trying to frighten you neither you have to take this warning literally. Instead it should be interpreted as that it's not a good thing at all to completely stop practicing the mantra your Guru has given you.
If you can remember the mantra now, you can start practicing it again. If you can't do that then in that situation the best person to guide you further is your Guru himself.