Since I have taken a year off from my Master's program due to a severe accident, all I am doing now are online courses as I am not fit to do any work. But will this in a way affect my future prospects in employment after I graduate? I feel really depressed as am not able to move due to spinal injury and heel fractures. Being an international student, I have to extend my student VISA as well. Moreover, a letter of support has been provided by my professor(he is the head of the Master's program) for the extension of VISA. Also, I had to withdraw from courses with academic penalty(but with refund of course fees) that will affect my academic standing which has indirect affect on VISA. How will it affect my future prospects of employability?
Your future is impossible to predict. But online courses, under the given circumstances seem to be a positive thing, indicating that you are keeping engaged with your studies. But use them well. Remember that just seeing lectures isn't the same as learning. Good online courses will provide ways for you to reinforce the learning (exercises). The best will find a way to give you individual feedback on your work, just as would occur in a face-to-face course.
But, I think your primary current responsibility is to regain your health, which will make all things possible.
The people for which gaps are a problem are those who have no reason for it other than lack of focus and commitment. You don't have any issues like that. Good luck and good health.