Name the delay before restarting the topic
Say that you saw or received their message and did not respond. If the original message contained a question, invitation, or vulnerable disclosure, address it rather than behaving as though the thread is new.
Give context without asking them to erase the impact
Briefly explain overwhelm, avoidance, illness, or changing circumstances if it helps, but do not make your friend reassure you. The missed reply may have felt dismissive even without that intent.
Make reconnection easy to decline
Say what you hope for now, such as a short call or renewed contact, and acknowledge that they may need time or may not want to resume the friendship.