I am working in the field of HCI. My relationship with my advisor turned sour due to difference of opinion and eventually I changed my advisor. Now I am stuck in a situation where my paper as a first author is on hold because the ex-advisor is not letting me submit it. This is primarily my work of 2 yrs., while my ex-advisor is a coauthor based on guiding me throughout the research, reading my writing, and giving his comments. What should I do in this situation?
This is quite a tricky situation. You can't really submit until they either approve or withdraw from the paper. I think, assuming you have asked them several times, then your best option may be to drop an email to your head of department. Stay very polite - emails often get forwarded on, and ask them for advice on what to do. I would think they will most likely forward that to your co-author (without you being on the email) and ask what they are playing at, and things will then move.
According to the comments, your ex-supervisor is a coauthor of the paper. The nature of their contributions likely makes it impossible to cut out parts of the paper to obtain a version they would not be considered a coauthor of.
You need the consent of all coauthors for submitting a paper.
How to proceed will depend on the nature of your ex-supervisors refusal.
Scenario A: They refuse based on objections to the research itself or its presentation.
Work under the assumption that their objections are genuine as much as possible, and address them. Do keep in mind the possibility that there could be a flaw in your research approach, maybe even a fatal one. Also keep in mind that publishing a paper with coauthor will on occasion involve making compromises in how to present stuff (and that having the paper published probably is more important than having everything presented just the way you'd prefer it).
If above assumption becomes difficult to sustain, consult another expert in the area (maybe your new supervisor). If that expert is very positive about the merits of the draft, ask them to support your case. (If you get a mere "well, I'd consider this draft publishable somewhere, I guess" - forget about the paper and move on.)
Scenario B: They explicitly refuse, in order to spite you.
I would consider this a very unlikely case, because it would not only be unprofessional to do this to a (former) student, but also be unwise to admit this. Here a complaint to their head of department seems in order.
Scenario C: They just don't consent, either giving you silence or delaying getting back to you forever.
Try to get someone both you and your ex-supervisor respect involved. Maybe your new supervisor, maybe a head of department, etc. The involvement you ask for is simply to ask your ex-supervisor for an update on your paper. Ignoring them will be more problematic than ignoring you, and you'll probably find yourself with either permission to submit, or a variant of Scenario A eventually.