Date and time on Lock Screen became grey

A few days ago, when temperatures went under 0 degrees, my phone suddenly stopped working and a message appeared saying: iPhone temporarily unavailable - try again in 1 minute. This issue lasted for just a few minutes after which iPhone was working again fine (and I think I also turned off and turned on the phone trying to resolve). However, since then the date and time which are shown on the Lock Screen are no longer white and became grey/transparent and are not clearly visible. Can someone tell me what is the issue and how can I resolve this and get date and time back to a white colour as it was before?

many thanks!