The AC21 law says that your I-485 has to be pending and unadjudicated for greater than 180 for you to switch jobs. The new job has to be same or similar.
However, the GC process is for future employment; thus the foriegn national does not have to be working for the sponsoring employer before granting the GC, only after.
Let me clarify:
1- Company A sponsors Mr. X for green card.
2- Mr. X does not have to work for Company A prior to GC approval; however, Company A must be willing to hire him and he must work for it after the GC approval. This is why, when the Legacy INS sends an RFE for letter of employment, it needs to state the intent of the company to hire the forien national after GC approval.
3- During the adjudication process Mr. X can work for any company at any time in any field. However, Company A must still maintain the intention to hire him at the end of the GC process.
4- AC21 came and changed that a little. It said that if the I-485 is pending more than 180 days, then Mr. X can find another Company B to sponsor his GC provided he will be working in the same or similar field with company B as he would have been with Company A.
5- Again untill the GC is approved Mr. X can for for anyone at any time. But the sponsoring Company A should have the intention to hire him at the end of the GC process.
All of this is build upon the intention of Company B to hire Mr. X after the GC is approved. To demonstrate that intention to the BCIS, generally the company hires Mr. X right away and he keeps the same job until the GC is approved.
So in your case, Company A is no loger in the picture (it disappeared). Now you need to find Company B to sposor your GC. That sponsor can only get in the picture with the BCIS after the I-485 pending 180 days, and it has to offer the same or similar job. But in the mean time you can work for any other Company C, D, E, F,.......
Hope I did not confuse you more,
Cheers,