HP 35s Scientific Calculator - Manual (Page 291)

Support, Batteries, and Service A-5
3. Remove the batteries (see "Changing the Batteries") and lightly press a
coin against both battery contacts in the calculator. Replace the batteries
and turn on the calculator. It should display  .
4. If the calculator still does not respond to keystrokes, use a thin, pointed
object to press the RESET hole. Stored data usually remain intact.
Reset Hole
If these steps fail to restore calculator operation, it requires service.
If the calculator responds to keystrokes but you suspect that it is
malfunctioning:
1. Do the self–test described in the next section. If the calculator fails the self
test, it requires service.
2. If the calculator passes the self–test, you may have made a mistake
operating the calculator. Reread portions of the manual and check
"Answers to Common Questions" (page A–1).
3. Contact the Calculator Support Department listed on page A–8.
The Self–Test
If the display can be turned on, but the calculator does not seem to be operating
properly, do the following diagnostic self–test.
1. Hold down the  key, then press  at the same time.
2. Press any key eight times and watch the various patterns displayed. After
you've pressed the key eight times, the calculator displays the copyright
message ©       and then the message  .
3. Press the keys in the following sequence: