Advice on shares after redundancy and merger

The company I worked for recently made me redundant. I have shares in the company which recently went through a merger and now my shares are worth half of what they were prior to the merger which was a stock-for-stock merger.

When I was given the shares I apparently had a physical share certificate but I have never been sent anything by the company regarding the shares (I do have them though as it’s part of another contract) and they have confirmed as such. These shares were issued prior to the merger.

Prior to the merger which happened after receiving my shares, I had no documentation from the company regarding the merger and what it would mean regarding the shares. The merger was announced via a press release last May and concluded in September.

My question is, could I have sold my shares between the announcement and the merger conclusion if I had been issued a physical or electronic share certificate when I was originally given the shares the year before and avoided losing half their value? Would I have needed to have been physically given a certificate by the company to allow me to do that?

I am trying to figure out if I had not been all the information and documentation that a company is required to do as a shareholder and I could have avoided the devaluation in the shares.

Thanks for any advice.