This topic was covered in the now-closed Tandem Alumni Forum  (operated by former Tandem employees, not by the HPAA.) 

If you have paper stock certificates. What to do: Stock Certificates

Keep address current on every financial account. Run easy unclaimed property search every year. Employer does not update your address. Neither cashing dividend checks, receiving direct deposits, online access, nor receiving statements prevents inactive accounts from being turned over to the state – only a personally-initiated transaction, postal response, or call. Difficult to retrieve money; stock is sold. Unclaimed Property

To be sure, also check for unclaimed stock or dividends in the original state of incorporation: Delaware

HP Inc is the overall legal successor to Compaq.  Stockholder information and contact instructions: HP Inc Investor Relations Site https://investor.hp.com/resources/contact-ir/default.aspx (scroll down)

Compare the date when you first acquired Tandem stock with the dates in the left-hand column. If you later acquired Compaq shares while a Compaq employee, those batches of stock should be tracked separately -- see our Compaq Stock Decoder.

For links and more detail on HP stock history, see our HP Stock Decoder

Note: Due to the many complex HP-related stock events, only you – not the company, plan administrators, transfer agents, brokers, or the IRS – can reconstruct your full stock history. Transactions were often tracked and reported differently – or not at all. Retain your documents and tax calculations indefinitely. Admin and Broker Issues

The black entries in the table below are from the online system available to customers of one broker, Charles Schwab & Co. The HPAA member who contributed this table did not hold this stock before 1987.

The blue entries were developed from reports by members of the Tandem Alumni Forum and contemporaneous news stories. Given Tandem's high growth rate, there must have been earlier stock splits. (EDGAR, the SEC's searchable online database, includes SEC filings from some companies back to 1994 and from all companies only back to May 1996.)

This table only covers HP/HPE stock events through 04/30/2019. See our HP Stock Decoder for subsequent HP/HPE-related stock events.