Short answer: Pachkhan (pratyakhyan) should be taken before drinking water or starting food. Once you have drunk water, that act is already done and cannot be brought under a later pachkhan.

What you can do now

• If it was by mistake: do a brief alochana/michchhāmi dukkadam, then take a fresh pachkhan for the remaining time/meal.
• For regular meals: take pachkhan now for the rest of that bhojan-sitting and follow it carefully.
• For vows that do not allow water (e.g., Upvaas or Chauvihar after sunset): drinking water has already broken that vow. You cannot validly take that same pachkhan for the current period; you may take a new pachkhan for the next appropriate period.
• For vows that permit water (e.g., where pani is allowed till sunset): you may take pachkhan now for the remaining period and observe it properly.

Traditions

• Both Shvetāmbara and Digambara traditions keep the same principle: take pachkhan first, then begin pana (drinking) or bhojan (eating). Wordings/mantras differ by tradition, but the sequence and intent are the same.

Essence

• Pachkhan is a conscious resolve taken beforehand. Keep the order: resolve first, act after. If a slip happens, do prāyaścitta (repentance) and reset the vow correctly.