Standby database cannot find archive log

I am running oracle database enterprise edition, single instance, version 19.9

I am testing dataguard with a small database. We had a memory issue over the weekend and the primary became inaccessible but did not crash then. I killed the pmon, resolved the memory issue, brought standby down and restarted the system again but I have this problem:

SQL> select PROCESS,CLIENT_PROCESS,THREAD#,SEQUENCE#,BLOCK#

 2 from v$managed_standby

 3 where process = 'MRP0' or client_process='LGWR';

PROCESS  CLIENT_P   THREAD# SEQUENCE#    BLOCK#

--------- -------- ---------- ---------- ----------

RFS    LGWR         1      1619     22774

MRP0    N/A         1      1596    0

The archive log file is on the file system in the primary database server:-