Yes, it is possible that your Raspberry Pi is stopping working and you are unable to connect via SSH because the DHCP lease has expired. When a DHCP lease expires, the device loses its IP address and can no longer communicate with other devices on the network.

To diagnose the problem, you can try the following steps:

1. **Check the DHCP lease time.** You can do this by running the following command:

```
sudo cat /etc/dhcpcd.conf
```

Look for the line that says `dhcp_lease_time`. This is the amount of time that your Raspberry Pi will keep its IP address before it expires.

2. **Try renewing the DHCP lease.** You can do this by running the following command:

```
sudo dhclient -r wlan0
```

This will release the current IP address. Then, run the following command to renew the lease:

```
sudo dhclient wlan0
```

3. **Check the network connection.** You can do this by running the following command:

```
ifconfig
```

This will show you a list of all of the network interfaces on your Raspberry Pi, along with their IP addresses. Make sure that the `wlan0` interface has an IP address.

If you are still unable to connect to your Raspberry Pi via SSH, you can try connecting to it using a serial cable. Once you are connected, you can troubleshoot the problem further.

Here are some possible solutions to the problem:

* **Increase the DHCP lease time.** If your DHCP lease time is too short, your Raspberry Pi may lose its IP address before it has a chance to renew it. You can increase the lease time by editing the `/etc/dhcpcd.conf` file and changing the value of the `dhcp_lease_time` setting.
* **Set a static IP address.** If you have a reliable network, you can set a static IP address for your Raspberry Pi. This will prevent it from losing its IP address when the DHCP lease expires. To set a static IP address, edit the `/etc/dhcpcd.conf` file and add the following lines:

```
interface wlan0
static ip_address=192.168.1.100
static gateway=192.168.1.1
static subnet_mask=255.255.255.0
```

Replace `192.168.1.100` with the IP address that you want to assign to your Raspberry Pi, and replace `192.168.1.1` with the IP address of your router.

* **Update the firmware.** If you are using an older version of the Raspberry Pi firmware, it is possible that there is a bug that is causing the DHCP lease to expire prematurely. You can update the firmware by running the following command:

```
sudo rpi-update
```

If you are still having problems, you can try searching for help on the Raspberry Pi forums or Stack Exchange.
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