Fuel gauge flashing

Good morning from Greece. I’m having a problem the last days in my 451 passion that I own. After a few kilometers, i make a stop for 5 minutes to take a coffe and when I’m back the car doesn't start. I turn the key to the first position and the fuel gauge starts flashing. All bars. The fuel tank was half filled that time. I forgot to mention that the car is MHD so the two indicators of ECO flashing together with the fuel gauge. I’m turning the key to position 2 I’m hearing the relay of the starter but the car refuse to start. After 15 minutes of trying eventually start. Any ideas what it may be? Thank you and excuse me for my English.